Shower knob temperature control issue
First time home buyer with absolutely zero plumbing experience.
We have a super cool shower in the primary bathroom; rain shower, body sprayers, and a detachable wand. There are 3 knobs; top knob changes the water source (rain, body sprayers, or wand), middle knob is the on-off, and bottom knob is supposed to be for temperature.
At first we thought the temperature knob was just stuck, but soon we realized there is something different about it. We noticed there seems to be an additional ring installed that keeps the water temp stuck in one place and unable to be adjusted.
We also noticed what may be a set screw (?) that is only on this knob, further supporting the idea that there is something keeping this knob stuck in place and unable to be adjusted.
There was nothing mentioned about this in our home paperwork. We are wondering if we can remove this ring ourselves or if we need to call in help or even attempt to reach out to the previous owner to figure out what this is and why it was not disclosed.
Any tips on what might be the issue here? Or if this is a set screw or something else? Ideas for how to remove this ?